1. Field of the Invention
The present invention relates to a fuel cell including an electrolyte electrode assembly and a pair of separators for sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes electrodes and an electrolyte interposed between the electrodes. Further, the present invention relate to a method of operating the fuel cell.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (electrolyte electrode assembly) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane (proton exchange membrane). The membrane electrode assembly is interposed between separators. The membrane electrode assembly and the separators make up a unit of the fuel cell for generating electricity. A predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
The fuel cell has a fuel gas flow field (fluid flow field) defined in each separator for passing the fuel gas therethrough to the anode and an oxygen-containing gas flow field (fluid flow field) defined in each separator for passing the oxygen-containing gas therethrough to the cathode. If necessary, a coolant flow field for passing a coolant therethrough is defined between the separators along the surfaces of the separators.
The electrolyte membrane is required to be appropriately humidified to keep a desired ion conductivity and reduce any structural damage which would otherwise be caused to the electrolyte membrane if it were unduly dried. However, since the reactant gases that are supplied to the fuel cell have low humidity, the electrolyte membrane tends to be dried at inlets of the reactant gas flow fields.
When the fuel cell generates electric energy, i.e., the reactant gases react with each other, the fuel cell produces water. Because the produced water is liable to stay at outlets of the reactant gas flow fields, the electrolyte membrane tends to be excessively humidified, i.e., tends to suffer flooding, at the outlets of the reactant gas flow fields. The flooding possibly causes an insufficient supply of reactant gases to the surfaces of the electrodes.
In an attempt to address the problem, U.S. Pat. No. 5,935,726 (prior art 1) discloses a method of and an apparatus for distributing water to an ion exchange membrane in a fuel cell. According to prior art 1, the direction in which an oxygen-containing gas flows through an oxygen-containing gas flow field is periodically reversed to prevent excessive drying of an electrolyte membrane in the vicinity of a gas inlet and also to prevent flooding in the vicinity of a gas outlet for thereby uniformizing a distribution of water in the fuel cell.
According to prior art 1, however, since a switching mechanism (solenoid-operated directional control valve or the like) is used to change the direction of the flow of the oxygen-containing gas, the flow of the oxygen-containing gas occasionally stops in the oxygen-containing gas flow field. Because of such occasions, the supply of the oxygen-containing gas becomes unstable, making it impossible for the fuel cell to keep a stable output of electric energy.
Japanese laid-open patent publication No. 2002-141086 (prior art 2) discloses a fuel cell in which the flow direction of an oxygen-containing gas can be switched according to the operating condition of the fuel cell. The oxygen-containing gas flows in the direction of anti-gravity in dry condition, and flows in the direction of gravity in wet condition. The amount of water produced in the reactions of the fuel cell is controlled by switching the flow direction of the oxygen-containing gas for preventing the dry up and flooding in the fuel cell.
According to prior art 2, however, the overall fuel cell needs to be reversed using a motor. Therefore, the fuel cell system is considerably complicated, and large.